1. Field of the Invention:
This invention relates to a process for reforming a surface of machinery and tools to be used under severe working conditions, such as a corrosive and high temperature atmosphere, as well as high stress, by an electric discharge process using a special electrode.
As has already been known, recycling vessels for nuclear fuels, vessels for chemical reactors, and so forth, are required to have high corrosion-resistant property; moreover, the blades and nozzles for gas turbine and steam turbine, and further, the jet nozzles for rocket engines, etc. are required to have high-temperature-oxidation-resistant property.
In order to improve the corrosion resistance of these structural elements, there have so far been attempted the use of a plating method and a chemical vapor deposition (CVD) method; and, for improving the heat resistant property, there has so far been adopted thermal spraying of a ceramic. All of these methods are, however, not satisfactory in their utilization in the above-mentioned fields.
As the measures for removing such disadvantages and attaining satisfactory improvement in the surface conditions of the machinery, etc., there may be contemplated reforming the surface into an amorphous structure or a very fine crystal structure; however, such method is not known. The amorphous structure which has been known and manufactured at present is limited only to very fine powder or a very thin plate, or a rod of very small size. Manufacture of such plate or rod having a fairly large size would accompany difficulty, and yet it has been considered impossible to impart to the surface of a certain base material such favorable properties, or to rigidly attach thereto such desirable amorphous structure, and hence improved properties.
Also, there has been known a method for carrying out reformation of the surface of the machinery, etc. by utilizing the electrical discharge phenomenon. For instance, in an attempt to harden the surface of a steel material by use of a tungsten electrode, electric current is caused to pass across the tungsten electrode and the surface of the steel material, while vibrating the tungsten electrode, to repeat the short-circuiting and the circuit-opening, thereby carrying out migration of the tungsten material to the steel material.
With the above-mentioned method of the surface reformation, however, no improvement in the corrosion-resistance and the high-temperature-oxidation resistance of the steel material could be attained, even though its surface becomes harder, because the surface of the steel material was not so compact.
As mentioned in the foregoing, there have so far been contemplated and attempted various methods for imparting to the surface of the metal materials high- corrosion-resistance and high-temperature-oxidation resistance being rigidly adhered to the base material, but no effective and decisive methods could be successfully realized.
The present invention has been made with a view to solving the problem as mentioned above, and aims at realizing the method of forming a surface layer having an amorphous or very fine crystal structure with high corrosion-resistant property and high heat-resistant property, by means of an electrical discharge process using a special electrode.
The method of forming the surface layer according to the present invention is to carry out an electrical discharge process on a work-piece by using a metalloid or a metal such as silicon, germanium, bismuth or zirconium as an electrode for such an electrical discharge process. The electrical discharge process is conducted in a liquid, i.e., a process liquid.
According to the method of the present invention, since the metalloid or the metallic electrode has high resistance, a multitude of very fine electrical discharges take place simultaneously on the entire surface of the electrode without causing such discharge to occur at one portion of the electrode surface, whereby the metalloid or the metallic electrode having low heat conductivity is evaporated and migrates onto the surface of a work-piece, because of its having electric charge. In this way, abrupt heating and cooling of the work-piece is effected at a high temperature and under a high pressure to thereby form the amorphous or very fine crystal structure on the surface of the work-piece.
Preferably, silicon, germanium, bismuth and zirconium is used as a positive electrode in the electric discharge process, and a potential of +300 mV or below is applied.
In order to further improve the surface roughness (nm R.sub.max) of the metalloid or metallic electrode, such electrode has a surface are of 15 cm.sup.2 or more. This contributes to the quality of the surface by maintaining the intensity of the electric current to be small and thus providing a smooth surface.
In the following, the present invention will be described with reference to a few preferred examples thereof in conjunction with the accompanying drawing.